For at least some pumping applications which employ centrifugal pumps, a vacuum source connected at or near the inlet of the pump is used to aid or effect priming of the pump at start-up or to maintain the priming of the pump during its operation. One application Of this type of centrifugal pump is in the pumping of sewage.
There are several methods for creating a source of vacuum for the centrifugal pump. One method utilizes an "eductor" which those skilled in the art will recognize as a device which uses high pressure air flowing through a venturi to create a source of vacuum. Another method of providing the required vacuum is the use of a vacuum pump. To effect priming of a pump, the vacuum source is placed in fluid communication with the pump inlet in order to remove air or other gases from the pump inlet. As the air/gas is removed, liquid is drawn into the inlet to replace the air/gas removed by the vacuum source; the centrifugal pump is then able to commence pumping of the liquid.
It should be apparent that it is undesirable for the pumpage to enter the vacuum system (often termed "carry over"). When a vacuum pump is used as a source of vacuum, it is easily damaged if the liquid being pumped, even if water, enters the pump.
Eductor-type systems are generally less sensitive to the ingestion of carry-over and may include apparatus for discharging carry-over drawn into the vacuum systems. If the carry-over is other than water, its discharge from the vacuum system may have to be carefully controlled. This is especially true of pumps used in sewage applications.
Efforts have been made to control the entry of liquid into the vacuum system. Many pumps of this type incorporate a float valve which closes off the vacuum passage when liquid in the priming chamber rises to a predetermined level. One suggested solution involves the use of peel valve which covers a flat plate containing ports communicating with the vacuum source, when the level in the priming chamber rises to a certain level.
It is believed that these prior efforts have not been entirely satisfactory. In actual operation, it has been found that surges occur in the fluid being pumped which cause abrupt changes in liquid level in the priming chamber. It is believed that the surging fluid can cause unseating of the vacuum valve and allow fluid from the priming chamber to enter the vacuum system. In some prior art pumps, provisions have been made for dealing with carry-over into the vacuum system by providing additional separating devices for recapturing the fluid entering the vacuum system and returning it to the pump. This adds unwanted expense to the pumping systems.
Vacuum assisted priming systems for pumps, by their design, allow pumps to lose prime due to lack of pumpage and then effect repriming of the pump when pumpage again becomes available. This periodic loss of prime is often encountered when pumps are used for drainage and like operations. Because of this repriming capability, pumps may run "dry" for extended periods of time. Most of these pumps have mechanical seals for inhibiting the leakage of pumpage out of the pumping chamber. These mechanical seals are subject to failure if they are allowed to overheat, which may occur when the pump is run dry. Attempts at solving this seal problem have been made in the past. One suggested solution is to use carbide facings for the seal and to immerse the external side of the seal in an oil bath. Some manufacturers have even gone to the step of creating circulation of the seal oil by using an elevated oil header tank operating on a thermo-syphon system that uses the density changes within the oil to generate a flow from hot to cool. These methods for cooling the seal have not been totally satisfactory.